I have a dataset that describes a few million incidents along Texas roadways. The vast majority of incidents only list a highway name and a milepost. For example, milepost 120 on I-20 or milepost 263 on US 60.

How would I convert these milepost/highways to latitude and longitude?

This is for a low-budget academic project, so trying to stick with free tools as much as possible.

My Google searching hasn't turned up much fruit, although some indications are that TxDOT may have data I could request through the Texas Public Information Act, but not sure yet.

If you have a table of points with:
HighwayName
Latitude
Longitude
Mile

Then it is a matter of interpolating lat/long between two adjacent rows using the Mile post value (safe to assume flat earth here). If you have historic data, keep in mind road re-alignments can sometimes result in milepoint changes. I think TxDOT tried to manage this using a method described in this paper, but was unsuccessful.

Thank you for the tip! I will check on Michael. I was able to get that article through my university's library. Thanks for suggesting it as the TxDOT system seemed confusing per the stuff I found in their online documentation.
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Aren CambreDec 2 '10 at 3:35

Per markusN, looks like GRASS GIS supports it, and Quantum GIS also has a LR module. We do have ArcGIS, but it's going to be a pain to get it on my laptop due to licensing. Plus I'd like to acquire GIS app skills that I can use anywhere, not just in context of the degree I'm working on, so hoping (naively?) that the FOSS packages may be sufficient?
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Aren CambreDec 2 '10 at 3:43

Thanks. I did download and review GRASS GIS a few weeks ago but moved to Quantum GIS as it appeared to have broader usage and actually includes GRASS GIS 6.4 as a package. But not sure Quantum GIS's LR module is as thorough as GRASS's.
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Aren CambreDec 2 '10 at 3:42